This invention relates to deterioration determination of a three-way catalyst which purifies exhaust gas from an internal combustion engine.
Tokkai Hei No. 7-63045 published by the Japanese Patent Office in 1995 discloses a device for determining the deterioration of a three-way catalyst which purifies exhaust gas of a V-type internal combustion engine wherein the engine cylinders are separated into two banks. The three-way catalyst is formed in an exhaust passage downstream of the confluence point of the exhaust gas discharged from the cylinders of each bank. The deterioration determination device is provided with two upstream oxygen sensors which detect the oxygen concentration in the exhaust gas for each bank, and a downstream oxygen sensor which detects the oxygen concentration downstream of the three-way catalyst. The air-fuel ratio of the fuel mixture supplied to the engine is controlled for each bank.
Namely, feedback control of the air-fuel ratio is performed for each bank so that a fuel mixture having the stoichiometric air-fuel ratio is supplied to the corresponding bank based on the output of the upstream oxygen sensor of each bank.
In air-fuel ratio feedback control, when the output of the downstream oxygen sensor does not follow the output inversion of the upstream oxygen sensor, it is determined that the three-way catalyst is not deteriorated.
On the other hand, when it follows the output inversion of the upstream oxygen sensor, it is determined that the three-way catalyst is deteriorated.
However, if the air-fuel ratio of one bank is shifted to rich and the air-fuel ratio of another bank shifts to lean at the same time, the output of the downstream oxygen sensor will stabilize at the stoichiometric air-fuel ratio, and the number of output inversions of the downstream oxygen sensor will decrease.
Consequently, even when the three-way catalyst has deteriorated, it may be determined that it has not deteriorated. In order to avoid such a diagnosis error, the prior art device prohibits deterioration determination of the three-way catalyst when the air-fuel ratio variation pattern of one bank is inverted from that of the other bank.
Tokkai Hei No. 6-193435 published by the Japanese Patent Office in 1994 discloses performing deterioration determination of the three-way catalyst based only, on the output of the downstream oxygen sensor in a V-type internal combustion engine wherein a three-way catalyst is provided for every bank.
Both of the above prior arts perform catalyst deterioration diagnosis in air-fuel ratio feedback control to the stoichiometric air-fuel ratio. In an engine which has few operation opportunities at the stoichiometric air-fuel ratio, as in the case of a direct-injection engine or a lean burn engine, if the deterioration determination is increasingly prohibited as in the former prior art, the opportunities for deterioration diagnosis of the catalyst will become less and less.
In the latter prior art, as the air-fuel ratio of each bank during catalyst deterioration diagnosis is controlled only according to the output of the downstream oxygen sensor, compared with the case where oxygen concentration and air-fuel ratio control is performed for each bank, the real air-fuel ratio of each bank tends to shift from the target stoichiometric air-fuel ratio. Such a shift of air-fuel ratio decreases the exhaust gas purification performance of the three-way catalyst with which each bank is equipped.
It is therefore an object of this invention to ensure opportunities for deterioration diagnosis of a catalyst while maintaining the precision of the diagnosis.
It is a further object of this invention to prevent a shift of a real air-fuel ratio from the target air-fuel ratio in deterioration diagnosis of a catalyst.
In order to achieve the above objects, this invention provides an air-fuel ratio control device for such an internal combustion engine that comprises a first exhaust passage connected to a first cylinder and provided with a first exhaust gas purification device which has an oxygen storage function, a second exhaust passage connected to a second cylinder and provided with a second exhaust gas purification device which has an oxygen storage function, and a collective exhaust passage which collects exhaust gas from the first exhaust passage and the second exhaust passage
The control device comprises a first fuel supply device which supplies fuel to the first cylinder to generate an air-fuel mixture to be burned in the first cylinder, a second fuel supply device which supplies fuel to the second cylinder to generate all air-fuel mixture to be burned in the second cylinder, a first oxygen sensor which reacts to an oxygen concentration in exhaust gas in the first exhaust passage upstream of the first exhaust gas purification device, a second oxygen sensor which reacts to an oxygen concentration in exhaust gas in the second exhaust passage upstream of the second exhaust gas purification device, a third oxygen sensor which reacts to an oxygen concentration in exhaust gas in the collective exhaust passage, and a programmable controller.
The controller is programmed to calculate a first feedback correction coefficient for controlling an air-fuel ratio of the air-fuel mixture in the first cylinder to a target air-fuel ratio based on a reaction of the first oxygen sensor, feedback control a fuel supply amount of the first fuel supply device using the first feedback correction coefficient, calculate a second feedback correction coefficient for controlling an air-fuel ratio of the air-fuel mixture in the second cylinder to the target air-fuel ratio based on a reaction of the second oxygen sensor, feedback control a fuel supply amount of the second fuel supply device using the second air-fuel ratio feedback correction coefficient, determine whether or not a predetermined condition for performing deterioration diagnosis of the exhaust purification devices is satisfied, and perform the deterioration diagnosis of the exhaust purification devices, when the predetermined condition is satisfied, based on a reaction of the third oxygen sensor while feedback controlling the fuel supply amount of the first fuel supply device and the fuel supply amount of the second fuel supply device using the first feedback correction coefficient.
This invention also provides a control method for such an internal combustion engine that comprises a first exhaust passage connected to a first cylinder and provided with a first exhaust gas purification device which has an oxygen storage function, a second exhaust passage connected to a second cylinder and provided with a second exhaust gas purification device which has an oxygen storage function, a collective exhaust passage which collects exhaust gas from the first exhaust passage and the second exhaust passage, a first fuel supply device which supplies fuel to the first cylinder to generate an air-fuel mixture to be burned in the first cylinder, and a second fuel supply device which supplies fuel to the second cylinder to generate an air-fuel mixture to be burned in the second cylinder.
The method comprises detecting a first oxygen concentration in exhaust gas ill the first exhaust passage upstream of the first exhaust gas purification device, detecting a second oxygen concentration in exhaust gas in the second exhaust passage upstream of the second exhaust gas purification device, detecting a third oxygen concentration in exhaust gas in the collective exhaust passage, calculating a first feedback correction coefficient for controlling an air-fuel ratio of the air-fuel mixture in the first cylinder to a target air-fuel ratio based on the first oxygen concentration:
feedback controlling a fuel supply amount of the first fuel supply device using the first feedback correction coefficient, calculating a second feedback correction coefficient for controlling an air-fuel ratio of the air-fuel mixture in the second cylinder to the target air-fuel ratio based on the second oxygen concentration, feedback controlling a fuel supply amount of the second fuel supply device using the second air-fuel ratio feedback correction coefficient, determining whether or not a predetermined condition for performing deterioration diagnosis of the exhaust purification devices is satisfied, and performing the deterioration diagnosis of the exhaust purification devices, when the predetermined condition is satisfied, based on the third oxygen concentration while feedback controlling the fuel supply amount of the first fuel supply device and the fuel supply amount of the second fuel supply device using the first feedback correction coefficient.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.